In recent years, a lithium ion battery is mounted as a rechargeable battery to portable equipment such as a digital camera. Since the lithium ion battery is weak in overcharge and overdischarge, a battery pack is provided with a protection circuit for overcharging and overdischarging in many cases.
FIG. 1 and FIG. 2 are block diagrams of conventional battery packs. In the battery pack 1 shown in FIG. 1, a series circuit containing a capacitor C1 and a resistor R1 is connected in parallel to a lithium ion battery 2. A positive terminal of the lithium ion battery 2 is connected to an external terminal 3 of the battery pack 1, and a negative terminal is connected to an external terminal 4 of the battery pack 1 through n-channel MOS (metal oxidization film semiconductor) transistors M1 and M2 for current interruption.
Drains of the MOS transistors M1 and M2 are connected in common. A source of the MOS transistor M1 is connected to the negative terminal of the lithium ion battery 2. A source of the MOS transistor M2 is connected to the external terminal 4. Body diodes D1 and D2 are connected equivalently between the source and drain of the MOS transistors M1 and M2, respectively.
A protection IC (integrated circuit) 5 incorporates therein an overcharge detection circuit, an overdischarge detection circuit and an overcurrent detection circuit. The protection IC 5 operates by a power supply Vdd supplied through the resistor R1 from the positive terminal of the lithium ion battery 2 and a power supply Vss supplied from the negative terminal of the lithium ion battery 2.
The protection IC 5 shuts off the MOS transistor M1 by causing a DOUT output to be a low-level when an overdischarge or an overcurrent is detected by the overcharge detection circuit or the overcurrent detection circuit. Additionally, the protection IC 5 shuts off the MOS transistor M2 by causing a COUT output to be a low-level when an overcharge is detected by the overcharge detection circuit.
The battery pack 1 shown in FIG. 2 is further provided with a thermistor R3. An end of the thermistor R3 is connected to a terminal 6 of the battery pack 1, and the other end is connected to an external terminal 4. A predetermined voltage is applied to the terminal 6 of the battery pack 1 through a voltage-dividing resistor from a charge device when charging. A voltage of the terminal 6 changes according to a resistance value of the thermistor R3 being changed by a temperature of the battery pack. The charge device, which controls the charge, causes the charge to stop by detecting the voltage at the terminal 6 when the temperature of the battery pack 1 exceeds a predetermined value.
Patent Document 1 discloses that a temperature protection element (PTC element) does not operate even if a high-temperature is generated during a time of normal discharge by connecting to a secondary battery a diode connected in series to the temperature protection element (PTC element) and a diode connected in parallel to the these and in a reverse direction.    Patent Document 1: Japanese Laid-Open Patent Application No. 2004-152560
The battery pack shown in FIG. 1 is not provided with a protection function to the temperature of the battery pack. The battery pack shown in FIG. 2 is provided with the protection function to the temperature of the battery pack. However, because a predetermined voltage is applied to the battery pack shown in FIG. 2 from the charge device through the voltage-dividing resistor, there is a problem in that the temperature of the battery pack cannot be detected accurately and an accurate charge stop control cannot be performed if the predetermined voltage of the charge resistor varies or if there is an error in the voltage-dividing resistor of the charge device.